Mirror vertex group (topology) isn't mirroring properly!
Suggestions


Inspect and Correct armature names. Make sure your armature has names such as


Hand.R and Hand.L  .

Copy the file for archive purposes, or duplicate scene. 3 minutes or less.
Edit to half body. Use Mirror Modifier Mesh. Make Perfect.
Apply Mirror Modifier. Full Body is result.  This may be viewed a common workflow.
Make no further changes to Body Mesh.
Perform quick and massive paint tests to see if new Body is weight paint friendly.